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Starting Over: RHS block foundation for Corvette LS7 Build

When Chad Reese downshifted his 2006 Z06 and planted his foot to the floor, his intention was to show the guy in the Cadillac CTS-V his taillights. Instead, he found himself rolling to the side of the road in an embarrassing cloud of smoke. His engine wasn’t cooperating and had decided to grenade itself instead.

Apparently, this is a bit of a common occurrence among these cars, more specifically the 2006-2014 Corvette Z06 coupes and 427 Convertibles powered with LS7s. The heads of their potent 7.0L engines did not receive proper machining from the supplier, and the valve guides are prone to failure — usually at the most inopportune time. Even though Chevrolet is working to correct this problem, the results are often disastrous, as evidenced by Chad’s piston that disintegrated into pieces no bigger than a pebble. It also windowed the block and contaminated the dry-sump oiling system, even sucking debris up into the intake manifold that could not be flushed out.

Disappointed, he decided to take action and rebuild. Since component failure is always an opportunity to upgrade, if Chad had to replace his engine, he might as well do it with one even better. His uncle also happens to work in the R&D area of COMP Cams. Together, they hatched a build plan to create an engine that was not only more powerful, but more durable as well.

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The Corvette LS7 build began with an LS Race Block and heads from RHS.

The foundation is a LS7 block and heads from RHS. The LS Race Blocks are made from aircraft-quality A357-T6 aluminum with press-in cast iron sleeves and a raised camshaft centerline. The heads have 12-degree valve angles and raised intake runners. They were fitted with 2.200-inch titanium intake and 1.615 stainless exhaust valves with 8mm stems. The stock 1.8-ratio rockers were treated to COMP’s Trunnion Bearing Upgrade kit and paired with springs rated for 505 lbs/in. The COMP lifters are short travel link bar-style, activated by ⅜-inch pushrods with a 0.135-inch wall.

The magic number of 427 for the displacement is reached with a bore and stroke of 4.125 x 4.000. A set of forged Mahle pistons with a 3cc valve relief was used and contributes to a compression ratio of 11.75:1. A Callies crank and 6.125-inch Lunati rods round out the rotating assembly.

With a factory intake full of piston gravel, a FAST LSXR 102mm intake was added as cheap insurance, versus trying to clean out the old one. Unlike the FAST version, the stock units are a one-piece design, making any attempts at flushing a daunting task. With a FAST 92mm throttle body bolted on, the engine was ready for the dyno.

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FAST’s LSXR 102mm intake manifold and 92mm throttle body were chosen in lieu of cleaning the stock counterparts.

Camshaft selection is everything in an engine, and getting useable power within a beneficial range is better than hitting max numbers. The first cam was pulled from COMP’s LS-based test bin, and it spec’d out at 247/257 with a 112 lobe separation. Installed at 109 degrees, it made an impressive 663.4 HP and 576.4 lb-ft of torque.

The peak horsepower was reached around 6.400-7.000 RPM, with the torque peaking from 5,400 to 5,500. There was also a curious dip in the torque curve within the 3,100 to 3,700 range.

While this is a significant increase over the car’s original 505 HP/470 lb-ft rating, the numbers were peaking in a lofty range more suitable to drag racing or an all-out dyno thrash, and the torque dip was right in a spot where it’s needed most. With goals of a street/road racer in mind, the crew knew they could dial the power back into a range more suited for that purpose.

They decided to come down a little bit on the intake lobe and widen the lobe separation. With a little less intake at 243/257 and a little more lobe separation at 115 (again, installed at 109), the results were a noted performance increase across the lower useable RPM areas, an elimination of the “dip” in torque, and even more peak torque. The torque line crested in a flatter curve between 4,700 and 5,600 RPM and then dropped off nearly the same amount as the baseline cam after that. The smaller intake lobe closes the intake sooner and shifts the torque curve up in the lower RPM range. The wider lobe separation makes for a broader torque curve and reduces overlap, which creates more vacuum and usually makes for a smoother idle, though this one didn’t idle smoothly. The added timing advance also increases lower RPM torque too.

The bottom-line horsepower number dropped from 663.4 to 654.1, but a look at the graph shows that happened between 6,500 and 7,000 rpm and at the expense of having an increase over the entire rest of the curve. This new torque and power range will be better suited to yanking a mass of a Corvette around through the gears in a street or road racing scenario.

While bigger numbers are always bragging points, if they don’t happen where you need them, it’s not really a win. At the end of the day, it’s torque that gets a car moving, and gaining in that area is always a home run.

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